1. Field of the Invention
This invention relates to bulk single crystal binary, ternary or quaternary metal nitrides such as gallium nitride, such metal nitrides being referred to broadly hereafter by the symbol M*N, including single crystal M*N substrate articles useful for formation of microelectronic structures thereon, as well as to an appertaining method of forming M*N in single crystal bulk form.
2. Description of the Related Art
The III-V nitrides, in consequence of their electronic and optical properties and heterostructure character, are highly advantageous in the fabrication of a wide range of microelectronic structures. In addition to their wide band gaps, the III-V nitrides also have direct band gaps and are able to form alloys which permit fabrication of well lattice-matched heterostructures. Consequently, devices made from the III-V nitrides can operate at high temperatures, with high power capabilities, and can efficiently emit light in the blue and ultraviolet regions of the electromagnetic spectrum. Devices fabricated from III-V nitrides have applications in full color displays, super-luminescent light-emitting diodes (LEDs), high density optical storage systems, excitation sources for spectroscopic analysis applications, etc. High temperature applications are found in automotive and aeronautical electronics.
To effectively utilize the aforementioned advantages of the III-V nitrides, however, requires that such materials have device quality -and a structure accommodating abrupt heterostructure interfaces, viz., III-V nitrides must be of single crystal character, substantially free of defects that are electrically or optically active.
A particularly advantageous III-V nitride is GaN. This nitride species can be utilized in combination with aluminum nitride (AlN) to provide optically efficient, high temperature, wide band gap heterostructure semiconductor systems having a convenient, closely matched heterostructure character similar to that of GaAs/AlAs. Indium nitride may also be added to GaN or AlN to provide additional advantages.
Corresponding advantages are inherent in ternary GaN compositions of the shorthand formula MGaN, wherein M is a metal compatible with Ga and N in the composition MGaN, and the composition MGaN is stable at standard temperature and pressure (25xc2x0 C. and 1 atmosphere pressure) conditions. Examples of potential M species include Al and In. Such compounds have compositions described by the formula M1-xGaxN, where x ranges from 0 to 1. The addition of a third compatible metal provides quaternary alloys of general formula M1-x-yMxe2x80x2yGaxN, where M and Mxe2x80x2 are compatible metals, in particular Al and In, and x and y range from 0 to 1. Such quaternary alloys are referred to by shorthand formula AlGaInN.
Alloys of GaN, AlN or InN with silicon carbide (SiC) may be advantageous because they can provide modulated band gaps. Such alloys have in the past been difficult to grow in single crystal form.
For ease of reference in the ensuing disclosure, therefore, the term xe2x80x9cM*Nxe2x80x9d is defined as including binary (e.g., GaN), ternary (MGaN), and quaternary (MMxe2x80x2GaN) type gallium nitride type compounds, as well as SiC, SiC/AlN alloys, SiC/GaN alloys, SiCInN alloys, and other related compounds such as alloys of SiC with AlGaN. All possible crystal forms are meant to be included in this shorthand term, including all cubic, hexagonal and rhombohedral modifications and all SiC polytypes. Examples of these compounds include AlN, InN, AlInN, AlGaN, InGaN, and AlInGaN.
For device applications, therefore, it would be highly advantageous to provide substrates of M*N, for epitaxial growth thereon of any of the M*N materials, especially GaN, AlGaN, InGaN, or SiC, for the production of heteroepitaxial devices. Unfortunately, however, it heretofore has not been possible to produce GaN in single crystal bulk form, and for all M*N materials, growth of high quality bulk single crystals has been fraught with difficulty.
It therefore would be a significant advance in the microelectronics art, and is correspondingly an object of the present invention, to provide M*N in bulk single crystal form, suitable for use thereof as a substrate body for the fabrication of microelectronic structures.
It is another object of the present invention to provide an appertaining method for the formation of bulk single crystal M*N which is relatively simple and may be readily achieved using conventional crystal growth techniques in an economic manner.
Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.
In one aspect, the present invention relates to a method of making a single crystal M*N article, including the steps of:
providing a substrate of material having a crystalline surface which is epitaxially compatible with M*N under the conditions of M*N growth;
depositing a layer of single crystal M*N over the surface of the substrate; and
etchably removing the substrate from the layer of single crystal M*N to yield the layer of single crystal M*N as said single crystal M*N article.
A key point of this invention is that the sacrificial substrate is etched away in situ, while the substrate/M*N structure is preferably at or near the growth temperature.
The sacrificial substrate may for example include a crystalline substrate material such as silicon, silicon carbide, gallium arsenide, sapphire, spinel (MgAl2O4), MgO, ScAlMgO4, LiAlO2, LiGaO2, ZnO, or a non-crystalline substrate of a material such as graphite, glass, M*N, SiO2, etc., for which a suitable etchant may be employed to remove the sacrificial substrate by etching. In the case of silicon and gallium arsenide, for example, HCl gas may be usefully employed. Additional substrates include silicon-on-insulator (SOI) substrates, compliant substrates of the type disclosed in U.S. Pat. No. 5,563,428 to B. A. Ek et al., and substrates containing buried implant species, such as hydrogen and/or oxygen. As a further alternative for the sacrificial substrate on which the M*N is grown, twist-bonded substrate structures may be used, i.e., where the substrate of crystalline material is bonded to another single crystal substrate material with a finite angular crystallographic misalignment. As yet another alternative, the substrate of crystalline material may be bonded to a suitable material, which preferably can be easily removed or has a similar thermal coefficient of expansion as the M*N.
The layer of single crystal M*N may be deposited directly on the surface of the crystalline or non-crystalline substrate, or alternatively it may be deposited on an uppermost surface of one or more intermediate layers which in turn are deposited on the crystalline substrate. The one or more intermediate layers may serve as a buffer layer to enhance the crystallinity or other characteristics of the M*N layer, as a template for the subsequent M*N growth thereon, or the intermediate layer(s) may serve as protective layer(s), or as an etch stop to prevent the etchant for the sacrificial substrate from etching into the M*N material.
When the substrate material has a protective layer or template layer deposited thereon, such layer is deposited on the substrate prior to growth of the M*N layer on the substrate, but such layer, or other intermediate layer(s), may be formed in situ in the growth chamber prior to initiation of growth of M*N thereon.
The growth of the M*N material may be carried out in a hydride vapor phase epitaxy (HVPE) reactor. As mentioned, protective layer may be employed to prevent decomposition of the single crystal substrate surface while M*N growth is proceeding. Such protective layer should be of a material favorable for M*N deposition. Examples include materials such as M*N and alloys thereof, wherein the protective layer is of a different material than the main body of the substrate, or is otherwise differently formed on the main body of the substrate, e.g., under different process conditions. The protective layer may be formed by any suitable technique such as for example sputtering, chemical vapor deposition, molecular beam epitaxy (MBE), vapor phase epitaxy (VPE),etc.
In another aspect, the invention utilizes the outdiffusion of specific species from the substrate into the M*N layer to provide enhanced properties of the final M*N product. An example of this aspect is the growth of M*N on a silicon substrate. In this case, Si can be caused to diffuse out of the silicon substrate and into the M*N. This diffusion will form a thin M*N region which is heavily doped with silicon. Silicon-doped M*N is n-type, and this structure is advantageous in certain device structures, as for example for making ohmic contacts to the back surface of the M*N layer or for forming p-n junctions.
In another aspect, the invention may be carried out with a substrate which has been implanted with hydrogen, whereby during the deposition of M*N or thereafter (during an elevated temperature exposure separation step) hydrogen builds pressure in situ in the heated substrate which in turn effects fracture of the substrate from the M*N material, to yield the M*N product article.
In another aspect, the invention relates to bulk single crystal M*N articles, such as are suitable for use as substrates for the fabrication of microelectronic structures thereon. As used herein, the term xe2x80x9cbulk single crystal M*Nxe2x80x9d refers to a body of single polytype crystalline M*N having three dimensional (x,y,z) character wherein each of the dimensions x, y is at least 100 micrometers and the direction z is at least 1 xcexcm. In the preferred practice of the invention, the single crystal M*N product will be of cylindrical or disc-shaped form, with diameter d and thickness z, where d is at least 100 xcexcm and z is at least 1 xcexcm. In a preferred aspect, each of the dimensions d and z is at least 200 micrometers. The bulk single crystal M*N article may most preferably have a thickness dimension z of at least 100 micrometers, and diameter dimension which is at least 2.5 centimeters.
Other aspects, features and embodiments of the invention will be more fully apparent from the ensuing disclosure and appended claims.